The Early Days of a Better Nation

Tuesday, February 24, 2009



Junkyard genes

I've already cited Richard Dawkins' review of Jerry Coyne's Why Evolution is True, and now I'm going to do it again, because it has (ha!) a genomics connection:
The molecular genetics revolution, which began in 1953, would have taken Darwin’s breath away and filled him with exultation. Every living creature carries within each of its cells a voluminous textual recipe for making itself. Nowadays, we can read these messages, accurately and with a completeness that is limited only by (rapidly shrinking) costs and time. Because the DNA texts of all animals and plants use the identical four-letter code, we have a gold mine of opportunity for comparison. In his own time, Darwin could compare, say, the wing of a bat, the flipper of a whale and the spade of a mole, and spot the relationships among a handful of bones. Today – and more cheaply in the near tomorrows – we can do it on an altogether grander scale, lining up billion-letter DNA texts from bat, whale and mole, and literally counting the single-letter discrepancies and resemblances. Moreover, we don’t have to limit our comparisons to one group, such as the mammals. The universal genetic code allows us to make letter-for-letter textual comparisons across plants, snails and bacteria, as well as vertebrates. This not only provides evidence for the fact of evolution that is orders of magnitude more solid even than the powerful evidence Darwin could muster. We can also construct, finally and definitively, the complete tree of all life, the universal pedigree. And we can find, in huge numbers, the molecular equivalents of vestigial evolutionary relics like the human appendix and the kiwi’s wings.

For the genome is littered with dead genes. Huge wastes of DNA territory comprise a graveyard of discarded, superseded old genes (plus meaningless sequences of nonsense DNA that never functioned) with occasional islands of current, extant genes that are actually read by the translating machinery and turned into action. Dead, untranslated genes are called pseudogenes. The reason our sense of smell is poor, compared with, say, that of dogs, is that most of our ancestral genes for smelling have been rendered inactive. We still have them, but they are dead. Molecular biologists can still read them – serried ranks of molecular “fossils” – but the body does not.

It is wonderful enough that we can construct a tree of life based on active genes, and find that different genes agree on the same pedigree. It is even more convincing that we get the same pedigree with dead genes, whose DNA sequences represent nothing, and must be regarded only as the inert legacy of history. How would creationists explain that? How would they explain the very existence of pseudogenes? Why would the creator litter the genome with useless, untranslated variants of genes, and locate them, moreover, in exactly the right pattern around the animal and plant kingdoms to give the impression – the deceptive impression, as a creationist would presumably have to admit – that they evolved and were not created?
This is all true, of course, but as always there are devils in the details: via The Panda's Thumb, here's a lucid explanation of why gene trees and species trees are not necessarily quite the same.

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2 Comments:

Now all we need is access to the great evolutionary version control system! Wait... You're telling me God didn't use a VCS? Oh, damn!

Have no fear, Max, the greatest trick God ever pulled was convincing the world he didn't exist. What do you mean 'why?' Oh. I see. Damn you, Dawkins!

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